Climate change is not a future problem for faraway places; it's affecting Americans now. This comes from a U.S. government report out today. NPR's Elizabeth Shogren says it also shows that choices people make now will have big ramifications for future generations.

ELIZABETH SHOGREN, BYLINE: The National Climate Assessment is the government's take on the latest science about climate change. This is the third one and its message is clear.

JOHN HOLDREN: Climate change is not a distant threat. It is already affecting every region in the country and key sectors of the economy.

SHOGREN: John Holdren advises President Obama on science. He says it's the loudest alarm yet that climate change is upon us. One of the biggest consequences is rising sea levels. Low-lying coastal communities, such as Miami, Norfolk and Portsmouth, New Hampshire, are most vulnerable. Ecologist Jerry Melillo chairs the government committee that drafted the report.

JERRY MELILLO: These places are experiencing the impact of sea level rise already in very serious and expensive ways.

SHOGREN: The report predicts additional sea level rise of one to four feet by the end of the century. Another big impact, longer periods of high temperatures and drought, especially in the Southwest. A third impact is more intense rain and snow storms. This affects the whole country but is felt mostly in the Midwest and Northeast. The report says it's too late to prevent many effects. One of the report's authors, University of Arizona professor Jim Buizer, says many communities are already aware of that.

JIM BUIZER: People are already starting to act, preparing for the climate that will be not the climate that was.

SHOGREN: Some communities are restoring coastal wetlands as buffers against storm surges. Others are restricting new construction in low-lying areas and changing building codes to, say, keep electrical equipment out of basements. Not surprisingly, many communities making investments are the ones that have suffered already. For instance, in Vermont a few years ago, rivers swollen by a big storm knocked out bridges. Vicki Arroyo helped the state adapt. She directs the climate center at Georgetown University.

VICKI ARROYO: They've spent millions and millions of dollars on changing their infrastructure, building larger culverts, building their bridges differently, because they know that storms of the future will often carry more water, like Hurricane Irene did.

ARROYO: Infrastructure has been built with the last 100 years in mind, not really looking ahead at these truly major changes we see on the horizon.

SHOGREN: Those major changes could be costly, but the report doesn't delve heavily into the economics. Robert Stavins thinks he knows why. He's an economist at Harvard.

ROBERT STAVINS: There's huge uncertainty regarding the nature of the impacts and also, therefore, regarding the economics of the impacts, where the uncertainty becomes even greater.

SHOGREN: People are going to have to do all sorts of things to adapt - build sea walls, find water for agriculture and adopt new sources of energy. All this is going to cost a lot of money. It's not clear Americans are ready to pay those bills. Public opinion polls consistently show that climate change is not a high priority for most Americans. Stavins says climate change is slow. Historically, it's taken an obvious disaster like a river catching fire to get Americans to pay to fix environmental problems.

STAVINS: But climate change is different. None of us observe the climate. We observe the weather. And even the abrupt climate change that scientists discuss is very, very gradual compared to the variance we have day to day and year to year on what the weather is.